Device and method for controlling the nox regeneration of a nox storage catalyst

ABSTRACT

The invention relates to a method and device for controlling the NO x  regeneration of a NO x  storage catalyst ( 14 ) disposed in the exhaust gas train ( 12 ) of an internal combustion engine ( 10 ) of a motor vehicle. The NO x  regeneration is at least initiated when a threshold value is exceeded for a load state of a NO x  storage catalyst ( 14 ) or a NO x  emission downstream from the NO x  storage catalyst ( 14 ). According to the invention, it is detected (a) whether the internal combustion engine ( 10 ) is idling and (b) alternately or in any possible combination, the threshold value for the load state or the NO x  emission is increased; the NO x  regeneration is only initiated after a specific amount of time has elapsed and a current NO x  regeneration is interrupted when a shift occurs into an idling mode.

BACKGROUND OF THE INVENTION

The invention relates to a method and device for controlling the NO_(x)regeneration of a NO_(x) storage catalyst located in the exhaust gaschannel of an internal combustion engine of a motor vehicle and havingthe features recited in the preambles of the independent claims.

It is known to integrate an exhaust gas cleaning device in the exhaustgas channel for the purpose of cleaning the exhaust of internalcombustion engines. The exhaust gas cleaning device typically includescomponents such as a particle filters or catalysts. If a raw emission ofNO_(x) of the internal combustion engine is to be reduced, then thesecatalysts include a reduction catalyst. If the mass flows of reducingpollutants, such as carbon monoxide CO and incompletely combustedcarbohydrides CH are sufficiently large in the region of the reductioncatalyst, then the reducing agent NO_(x) promotes a conversion tonitrogen.

To minimize fuel consumption, it has proven to be advantageous tooperate the internal combustion engine under the lean air conditions.However, the operation in the range optimized for fuel consumption isassociated, on one hand, with increased NO_(x) emission and, on theother hand, with reduced mass flows of reducing agents. To prevent highNO_(x) emission levels, a NO_(x) storage device is associated with thecatalyst which absorbs the NO_(x) in form of a nitrate. The NO_(x)storage device can be combined with the catalyst as a so-called NO_(x)storage catalyst.

The mass storage capacity of the NO_(x) storage catalyst is, of course,limited, so that a NO_(x) regeneration has be performed in regularintervals to prevent NO_(x) breakthroughs. During the NO_(x)regeneration, the operating mode changes to stoichiometric or rich. TheNO_(x) that had been absorbed in the form of nitrate is therebyreleased. Typically, a NO_(x) regeneration is initiated when a thresholdvalue for a load state of the NO_(x) storage catalyst or a NO_(x)emission detected downstream by a NO_(x)-sensitive measuring device(breakthrough emission) is exceeded. This has the disadvantage thatidentical criteria are applied for all operating phases of the motorvehicle to determine the need for regeneration. However, if the NO_(x)regeneration is initiated in the idle phase, significantly less exhaustgas flows and therefore the mass flows of reducing agents that arepresent are also smaller, so that the desorbed NO_(x) can only beincompletely reduced on the catalyst component. NO_(x) regenerationduring the idle phase does not only result in an undesirably high NO_(x)emission, but also leads to increased fuel consumption as compared toother operating phases where the internal combustion engine runs underhigher load. Disadvantageous, the NO_(x) regeneration in the idle phaseis frequently also accompanied by generation of undesirable noise.Moreover, NO_(x) regeneration under idle conditions takes longer due tothe smaller exhaust gas flows, and the operating conditions withunfavorable fuel consumption must be maintained for a longer time.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method and adevice which can obviate the aforedescribed disadvantages of theconventional technology. The obtained solution should be easilyintegratable with proven process control models.

The object is solved according to the invention by the device and methodfor controlling the NO_(x) regeneration of the NO_(x) storage catalysthaving the characterizing features recited in the independent claims.For example, a lean phase in idle mode can be extended to the nextabsolutely essential NO_(x) regeneration or can be controlled with thismethod according to the predetermined time intervals by:

-   -   (a) determining if an internal combustion engine is switched        into an idle mode, and    -   (b) alternatively or in any combination        -   increasing in the idle mode the threshold value for the load            state or the NO_(x) emission,        -   initiating the NO_(x) regeneration after predetermined time            intervals have passed, and        -   interrupting a current NO_(x) regeneration when changing            into the idle mode.

The device according to the invention includes means for carrying outthe aforedescribed method steps. Such means is preferable a controldevice in which a procedure is stored in digitized form which enablescontrol of the NO_(x) regeneration in idle mode. The control device canbe implemented as an independent control unit or can be integrated intoan often already existing engine controller.

If a NO_(x) regeneration is performed during a change into the idlemode, then the NO_(x) regeneration is preferably completed, if thechange into the idle mode occurs in a fuel-cutoff phase, if a rotationspeed exceeds a predetermined threshold value or a motor vehicle speedalso exceeds a predetermined limit speed. If the NO_(x) regeneration isinterrupted, then a marker is set which causes the NO_(x) regenerationto continue in a subsequent acceleration phase. Of course, the marker isremoved if a NO_(x) regeneration had to be already performed in the idlemode.

Preferably, the NO_(x) regeneration is performed by setting a lambdavalue in the range between 0.85 to 1.0. In any event, the NO_(x)regeneration should be performed under less rich conditions than isotherwise typical for NO_(x) regenerations. In this way, the noise levelcan be reduced in comparison to the “normal” NO_(x) regeneration atlambda values that are typically significantly less than 0.85. Accordingto another preferred embodiment of the method, a NO_(x) regeneration inidle mode is always initiated if for any reason a change into a λ=1operation is required. This can be the case, for example, when thepressure in a brake booster should be increased.

Generally, the aforedescribed procedures can reduce the number of NO_(x)regenerations in idle mode as compared to the other operating phases ofthe motor vehicle, so that fuel consumption, NO_(x) emission during theNO_(x) regeneration, as well as the noise generation are reduced.

Additional preferred embodiments of the invention are disclosed asadditional features in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in detail with reference to anembodiment illustrated in the appended drawings. It is shown in:

FIG. 1 a schematic diagram of an internal combustion engine with aNO_(x) storage catalyst arranged in the exhaust gas channel, and

FIG. 2 a schematic block diagram for controlling a NO_(x) regenerationof the NO_(x) storage catalyst in idle mode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an internal combustion engine 10 with a NO_(x) storagecatalyst 14 arranged downstream in the exhaust channel 12. Associatedwith the exhaust gas channel 12 is a suitable sensor circuitry formeasuring the air conditions in the exhaust or the fractions of specificpollutants. For example, a gas sensor 16 can be provided as a lambdaprobe or a gas sensor 18 as a NO_(x)-sensitive measuring device. Thedata measured by the sensor circuitry are supplied in a known manner ina motor controller 20. Models are stored in the motor controller 20 indigitized form for determining regulated values for the componentsassociated with the internal combustion engine 10. The components enablecontrol of an air-fuel ratio, an ignition angle or an injected fuelquantity in the combustion process. For example, regulated values canrepresent an opening angle of an exhaust return valve 22 or a positionof a throttle 24. The device and the method for regulating thecombustion process are sufficiently known and are therefore notdescribed in detail at this place.

In addition, other status parameters, for example a flap position of athrottle or a gas pedal angle are inputted into the motor controller 20,which can be used to determine in a known manner if the motor vehicle isidling. The status of the motor vehicle is subsequently read into acontrol device 36 which is implemented in the motor controller 20.

If an excess of oxygen is present during the combustion process of anair-fuel mixture, a raw emission of NO_(x) of the internal combustionengine 10 increases, while at the same time the quantity of reducingagents carbon monoxide CO and incompletely combusted carbohydrides CH,which are required for converting NO_(x), decreases. Since thisoperating range has proven to have a particularly advantageous fuelconsumption, the NO_(x) has to be absorbed in a storage component of theNO_(x) storage catalyst 14 to prevent NO_(x) emission. If the operatingmode changes into a stoichiometric or rich mode, then the NO_(x), whichis stored in form of nitrate, is desorbed again very quickly, at leastimmediately after the atmosphere in the NO_(x) storage catalyst 14changes. If the mass flows of reducing agents are too low, then thereducing agents cannot be supplied in sufficient quantity to thecatalyst component of the NO_(x) storage catalyst, which can causeundesirable NO_(x) emission.

With the method described hereinafter (see FIG. 2), the lean operatingmode can be maintained longer under idle conditions which typically havelower exhaust gas flows, which reduces the number of NO_(x)regenerations in the idle mode in comparison to other operating phases.Moreover, noise generation caused by the NO_(x) regeneration can besuppressed.

First, it is determined in an initial query if the motor vehicle isidling (step S1). If this is not the case, then the NO_(x) regenerationof the NO_(x) storage catalyst 14 can be controlled by a conventionalprocess. For example, a load state of the NO_(x) storage catalyst 14 ora NO_(x) emission downstream of the NO_(x) storage catalyst 14 can bemonitored (step S2). If this quantity exceeds a threshold value, theNO_(x) regeneration is initiated by changing into the stoichiometric orrich operation.

If the motor vehicle is idling, then it is determined in a subsequentquery (step S3), if the change into the idle mode occurs during anongoing NO_(x) regeneration. If this is affirmative, then it isdetermined in step S4, if a fuel cutoff phase exists and/or if the motorvehicle still has a speed above a predetermined limit speed, and/or if arotation speed exceeds a predetermined threshold value. If theseboundary conditions are fulfilled, then the NO_(x) regeneration is firstcompleted (step S5). Otherwise, the current NO_(x) regeneration isinterrupted and a marker is set (step S6). Setting this marker ensuresthat the NO_(x) regeneration is resumed at the end of the idle phase,for example in a subsequent acceleration phase of the motor vehicle.

After the steps S5and S6or if the change into the idle mode does notoccur during an ongoing NO_(x) regeneration (step S3), new thresholdvalues for determining the need for regeneration are set (step S7). Thethreshold values which are used with conventional processes for the loadstate and/or the NO_(x) emission are then increased. It will beunderstood that the values have to be set in a manner that nosignificant NO_(x) breakthroughs can occur during in idle mode, whichdue to the low exhaust gas mass flows can be guaranteed even if thethreshold values are higher than for the other operating phases of theinternal combustion engine 10.

As an alternative to the latter approach, a fixed time interval can beset in step S7, wherein the NO_(x) regeneration has to be performed atthe end of this time interval. Advantageously, in addition to theaforedescribed approach for controlling the NO_(x) regeneration in idlemode, the airfuel ratio during the NO_(x) regeneration can be set to avalue in a range of λ=0.85 to 1.0, and at least less rich than isotherwise typical for NO_(x) regenerations, which reduces the noisegeneration.

LIST OF REFERENCE NUMERALS

-   -   10 internal combustion engine    -   12 exhaust gas channel    -   14 NO_(x) storage catalyst    -   16 gas sensor    -   18 gas sensor    -   20 engine controller    -   22 exhaust gas return valve    -   24 throttle flap    -   36 control device

1. Method for controlling a NO_(x) regeneration of a NO_(x) storagecatalyst (14) arranged in an exhaust gas channel (12) of an internalcombustion engine (10) for motor vehicles, wherein the NO_(x)regeneration is initiated at least when a threshold value for a loadstate of the NO_(x) storage catalyst (14) or a threshold value for aNO_(x) emission downstream of the NO_(x) storage catalyst (14) isexceeded, the method comprising the steps of: (a) determining if theinternal combustion engine (10) is operating in an idle mode, and (b)performing at least one of the following steps for extending a leanoperation in the idle mode if the determination is that the engine isoperating in the idle mode: increasing the threshold value for the loadstate of the NO_(x) storage catalyst, increasing the threshold value forthe NO_(x) emission, initiating the NO_(x) regeneration after apredetermined time interval has passed, and interrupting a currentNO_(x) regeneration.
 2. Method according to claim 1, wherein a currentNO_(x) regeneration is not interrupted when changing into the idle mode,if a fuel cut-off phase occurs, if a motor vehicle speed exceeds apredetermined limit speed or if a rotation speed exceeds a predeterminedthreshold value.
 3. Method according to claim 2, wherein if a currentNO_(x) regeneration is interrupted, a marker is set which causes theNO_(x) regeneration to be continued in a subsequent acceleration phaseof the motor vehicle.
 4. Method according to claim 3, wherein the markeris removed if a NO_(x) regeneration already had to be performed in idlemode.
 5. Method according to claim 1, wherein the NO_(x) regeneration isperformed in idle mode by setting a lambda value in a range of 0.85 to1.0, but in any event less rich than is otherwise typical for NO_(x)regeneration.
 6. Method according to claim 1, wherein the NO_(x)regeneration is initiated during the idle mode when a change into a λ=1operation is required.
 7. Device for controlling a NO_(x) regenerationof a NO_(x) storage catalyst (14) arranged in an exhaust gas channel(12) of an internal combustion engine (10) for motor vehicles, whereinthe NO_(x) regeneration is initiated at least when a threshold value fora load state of the NO_(x) storage catalyst (14) or a threshold valuefor a NO_(x) emission downstream of the NO_(x) storage catalyst (14) isexceeded, wherein the device comprises a means for determining if aninternal combustion engine (10) is operating in an idle mode, andwherein, if the means determines that the engine is operating in theidle mode, the means performs at least one of the following steps forextending a lean engine operation in the idle mode: increasing thethreshold value for the load state of the NO_(x) storage catalyst,increasing the threshold value for the NO_(x) emission, initiating theNO_(x) regeneration after a predetermined time interval has passed, andinterrupting a current NO_(x) regeneration.
 8. Device according to claim7, wherein the means comprises a control device (36), in which controldevice a procedure is stored in digitized form for controlling theNO_(x) regeneration of the NO_(x) storage catalyst (14) in idle mode. 9.Device according to claim 8, wherein the control device (36) isintegrated in an engine controller (20).